The present invention relates to an optical substrate inspection apparatus for inspecting masks (reticules) or wafers having patterns requisite for fabricating semiconductor devices and particularly, to an optical substrate inspection apparatus using multi-beam laser light.
As LSI chips have been increased in the level of integration and the size of capacity, the width of each circuitry line for semiconductor devices is significantly reduced. In the production of such semiconductor devices, a pattern original (mask, reticule) which carries a pattern of circuitry lines is commonly used in a reduced image exposure system, known as a stepper, for printing the pattern on a wafer.
It is however true that the pattern on a mask or a wafer often includes various defects occurring during the fabrication. Those defects may result in malfunction of semiconductor devices and also may decline the productivity. It is essential in the production of semiconductor devices to conduct a step for eliminating such defects or an inspection step for detecting the presence of defects before subjecting to a repair or amendment step.
For the purpose of detecting defects on the pattern, optical substrate inspection apparatuses are widely employed. For example, one of them is disclosed as theutomated photomask inspection apparatus in European Patent Application 0532927A2. Another is depicted in "Mask defect inspection method by data base comparison with 0.25-0.35 .mu.m sensitivity", Jpn. J. Appl. Phys., Vol.33 (1994).
Such a conventional apparatus is however unfavorable because its optical resolution is not of a desirable level and the sensitivity for detection of defects is too low to be accepted. More specifically, any conventional optical substrate inspection apparatus includes an optical system of not a super-resolution type but a common integral-illumination type where the resolution is expressed by .lambda./NA as proportional to the wavelength .lambda. and inverse proportional to the numerical aperture NA used for the inspection (with a field of view being illuminated in its entirety).
In that system, the wavelength to be used for the inspection has to be set to a minimum for having an optimum resolution. When the wavelength is minimized to e.g. 250 nm which entails low responsibility of a detector, the integral illumination may be used with a less intensity of illumination and the time for the detection will thus be increased.
Another technique for detecting the defects on the pattern is a laser scanning which can provide a proper amount of laser beam. However, the technique uses a single beam of laser light for scanning over a target area. For providing an acceptable rate of the detection, the scanning with the laser beam has to be carried out at a higher speed. Accordingly, the amount of light fallen on a detector will be lowered and the time for the detection will hardly be decreased.
In the conventional optical substrate inspection apparatuses, the optical resolution gained is rarely as high as desired and the sensitivity for detecting the defects will unfavorably be low. In case that a pattern for an advanced semiconductor device such as a one gigabit DRAM, is developed in the near future, it will hardly be inspected precisely by the conventional apparatus. In the technique of the conventional apparatuses, a shorter wavelength optical system may be exploited for improving the sensitivity for detecting the defects. The fact that the time for the detection is still long cannot be overcome.
It is an object of the present invention to provide an optical substrate inspection apparatus capable of inspecting any defect on a pattern for an advanced semiconductor device (such as a one gigabit DRAM) with a higher level of the optical resolution but not increasing the time for inspection.